When red-hot magma spews from a volcano, it is clearly flowing like a liquid. But before erupting, that same magma may have spent hundreds of thousands of years in a chunky state resembling cold porridge, a new study finds. The discovery suggests that hot, liquid magma pools may be useful as an indicator of impending eruptions.

Volcanic magma, the molten mix of rock and gas below Earth’s surface, leads a sort of double life: It spends some time as a hot liquid and some as a colder, rocky “crys...